userMenu: Close menu immediately when activating Lock

When locking the screen, the user menu is moved to the opposite
side. Unless we close the menu immediately without animation, the
menu will jump to the other side and fade out while the screen
shield slides down.

https://bugzilla.gnome.org/show_bug.cgi?id=684343

js/ui/userMenu.js

@@ -12,6 +12,7 @@ const Tp = imports.gi.TelepathyGLib;
 const UPowerGlib = imports.gi.UPowerGlib;
 const Atk = imports.gi.Atk;
 
+const BoxPointer = imports.ui.boxpointer;
 const GnomeSession = imports.misc.gnomeSession;
 const Main = imports.ui.main;
 const PanelMenu = imports.ui.panelMenu;
@@ -813,6 +814,7 @@ const UserMenuButton = new Lang.Class({
     },
 
     _onLockScreenActivate: function() {
+        this.menu.close(BoxPointer.PopupAnimation.NONE);
         Main.overview.hide();
         Main.screenShield.lock(true);
     },